Wind turbines and wind turbine rotor blades with reduced radar cross sections

ABSTRACT

Wind turbine rotor blades with a reduced radar cross sections include a shell having a leading edge opposite a trailing edge, a structural support member that supports the shell and is disposed internal the wind turbine rotor blade between the leading edge and the trailing edge and extends for at least a portion of a rotor blade span length, wherein the structural support member comprises fiberglass, one or more cavities internal the wind turbine rotor blade, and a lightweight broadband radar absorbing filler material disposed in at least one of the one or more cavities to provide the reduced radar cross section.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to GE Docket No. 252906, filed concurrentlyherewith on Jan. 11, 2012, which is fully incorporated herein byreference and made a part hereof.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to wind turbines and, morespecifically, to wind turbines and wind turbine rotor blades withreduced radar cross sections.

Wind power can be considered one of the cleanest, most environmentallyfriendly energy sources presently available, and wind turbines havegained increased attention in this regard. A wind turbine can include atower, generator, gearbox, nacelle, and one or more rotor bladescomprising a composite material. The rotor blades capture kinetic energyfrom wind using known foil principles and transmit the kinetic energythrough rotational energy to turn a shaft coupling the rotor blades to agearbox, or if a gearbox is not used, directly to the generator. Thegenerator then converts the mechanical energy to electrical energy thatmay be deployed to a utility grid.

Wind turbines can thus be placed in a variety of locations toeffectively help capture the energy of wind power where present. Theselocations can include both on-shore and off-shore locations and maypotentially be located in a wide variety of different topographical andgeological positions. However, some position-based restrictions mayinhibit the feasibility of placing wind turbines and certain locations.For example, radar stations and the like, such as those used at manyairports, utilize open areas to capture radar feedback over greatdistances to monitor various activities such as air traffic. Placingwind turbines near such radar stations may result in consistent oroccasional radar feedback due to the radar cross section of one or morecomponents of the wind turbines and thereby impede the monitoring ofspace on the opposite side of such wind turbines.

Accordingly, alternative wind turbines and wind turbine rotor bladeswith reduced radar cross section would be welcome in the art.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, a wind turbine rotor blade with a reduced radar crosssection is provided. The wind turbine rotor blade includes a shellhaving a leading edge opposite a trailing edge, a structural supportmember that supports the shell and is disposed internal the wind turbinerotor blade between the leading edge and the trailing edge and extendsfor at least a portion of a rotor blade span length, wherein thestructural support member comprises fiberglass, and one or more cavitiesinternal the wind turbine rotor blade. The wind turbine rotor bladefurther includes a lightweight broadband radar absorbing filler materialdisposed in at least one of the one or more cavities to provide thereduced radar cross section.

In another embodiment, a wind turbine rotor blade with a reduced radarcross section is provided. The wind turbine rotor blade includes a shellhaving a leading edge opposite a trailing edge and a structural supportmember that supports the shell and is disposed internal the wind turbinerotor blade between the leading edge and the trailing edge and extendsfor at least a portion of a rotor blade span length, wherein thestructural support member comprises carbon fiber. The wind turbine rotorblade further includes a plurality of stacked resistive layers disposedat one or more locations about the structural support member to providethe reduced radar cross section.

These and additional features provided by the embodiments discussedherein will be more fully understood in view of the following detaileddescription, in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplaryin nature and not intended to limit the inventions defined by theclaims. The following detailed description of the illustrativeembodiments can be understood when read in conjunction with thefollowing drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 is a perspective view of a wind turbine according to one or moreembodiments shown or described herein;

FIG. 2 is a perspective view of a wind turbine rotor blade according toone or more embodiments shown or described herein;

FIG. 3 is a cross section view of a fiberglass based wind turbine rotorblade according to one or more embodiments shown or described herein;

FIG. 4 is a cross section view of a carbon fiber based wind turbinerotor blade according to one or more embodiments shown or describedherein;

FIG. 5 is an exploded view of a plurality of stacked resistive layersaccording to one or more embodiments shown or described herein; and

FIG. 6 is an exploded view of an individual resistive layer according toone or more embodiments shown or described herein.

DETAILED DESCRIPTION OF THE INVENTION

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, all features of an actual implementation may not bedescribed in the specification. It should be appreciated that in thedevelopment of any such actual implementation, as in any engineering ordesign project, numerous implementation-specific decisions must be madeto achieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

When introducing elements of various embodiments of the presentinvention, the articles “a,” “an,” “the,” and “said” are intended tomean that there are one or more of the elements. The terms “comprising,”“including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.

Wind turbines and wind turbine rotor blades with reduced radar crosssections are disclosed herein. In particular, lightweight broadbandradar absorbing filler material and/or stacked resistive layers can beincorporated into wind turbine rotor blades to provide relatively broador targeted radar absorption. For example, the lightweight broadbandradar absorbing filler material can be disposed in one of a plurality ofcavities internal the wind turbine rotor blade to reduce the radar crosssection across a relatively broad spectrum without significantlyaffecting the weight or balance of the wind turbine rotor blade.Similarly, a plurality of stacked resistive layers can be incorporatedinto the wind turbine rotor blade at one or more locations such as thestructural support member or the shell. When stacked resistive layersare disposed at multiple locations, different locations may be tuned toreduce the radar cross section at different frequencies. Incorporatingone or more of these features can allow the same wind turbine rotorblade to be disposed in different geographical areas that use differentradar bands while still providing a reduced radar cross section.

Referring now to FIG. 1, a perspective view of a wind turbine 10 isillustrated. The wind turbine 10 can generally comprise a nacelle 14mounted on a tower 12. A plurality of wind turbine rotor blades 16 canbe mounted to a rotor hub 18 which can be connected to a main flangethat turns a main rotor shaft (not illustrated). The wind turbine powergeneration and control components can be housed within the nacelle 14.It should be appreciated that the wind turbine 10 illustrated in FIG. 1is provided for illustrative purposes only and not intended to limit theapplication of this disclosure to a specific wind turbine type orconfiguration.

Referring now to FIG. 2, a perspective view of a wind turbine rotorblade 16 is illustrated. The wind turbine rotor blade 16 can include ablade root 20 for mounting the wind turbine rotor blade 16 to a mountingflange (not illustrated) of the wind turbine hub 18 (illustrated inFIG. 1) and a blade tip 22 disposed opposite the blade root 20. The windturbine rotor blade 16 may comprise a pressure side 24 and a suctionside 26 extending between a leading edge 28 and a trailing edge 30. Inaddition, the wind turbine rotor blade 16 may include a rotor blade spanlength 32 defining the total length between the blade root 20 and theblade tip 22. The wind turbine rotor blade 16 can further comprise achord 34 defining the total length between the leading edge 28 and thetrailing edge 30. It should be appreciated that the chord 34 may vary inlength with respect to the rotor blade span length 32 as the windturbine rotor blade 16 extends from the blade root 20 to the blade tip22.

The wind turbine rotor blade 16 may define any suitable aerodynamicprofile. Thus, in some embodiments, the wind turbine rotor blade 16 maydefine an airfoil shaped cross-section. For example, the wind turbinerotor blade 16 may also be aeroelastically tailored. Aeroelastictailoring of the wind turbine rotor blade 16 may entail bending the windturbine rotor blade 16 in generally a chordwise direction. The chordwisedirection generally corresponds to a direction parallel to the chord 34defined between the leading edge 28 and the trailing edge 30 of the windturbine rotor blade 16. Additionally, the spanwise direction generallycorresponds to a direction parallel to the rotor blade span length 32 ofthe wind turbine rotor blade 16. In some embodiments, aeroelastictailoring of the wind turbine rotor blade 16 may additionally oralternatively comprise twisting the wind turbine rotor blade 16, such asby twisting the rotor blade 16 in generally the chordwise directionand/or the spanwise direction.

Referring now to FIGS. 3 and 4, the cross sections of wind turbine rotorblades 16 are illustrated. The structure of the wind turbine rotor blade16 can generally comprise a shell 40 and a structural support member 50disposed within the shell 40. As illustrated in FIGS. 2 and 3, the shell40 can comprise the leading edge 28 opposite the trailing edge 30. Theshell 40 can comprise any material that allows for the capture ofincoming wind for rotating the wind turbine rotor blade 16 while beingable to be supported by the structural support member 50. For example,in some embodiments the shell 40 can comprise a composite material. Insome embodiments, the shell 40 can comprise a fiberglass material or acarbon fiber material. In even some embodiments, the shell 40 cancomprise a plurality of layers (e.g., a plurality of fiberglass layers)that are connected to one another through adhesives (e.g., glues, tapes,etc.), mechanical fasteners (e.g., screws, bolts, etc.) or the like.While specific embodiments of wind turbine rotor blades 16 have beendisclosed herein, it should be appreciated that these embodiments arenot intended to be limiting and alternative wind turbine rotor blades 16(e.g., using additional and/or alternative materials, designs or thelike) should also be appreciated.

In some embodiments, the shell 40 can comprise a plurality of layersheld together by an adhesive such as an epoxy adhesive. In suchembodiments, the amount of adhesive, or similar binder, may vary inamount by location. Thus some locations of the shell 40 may comprise agreater amount (e.g., a thicker amount) of adhesive as a result of themanufacturing and/or assembly process. Furthermore, the adhesive mayalso interact with radar such that the variations in amounts of adhesivemay lead to variations in its radar cross section along the wind turbinerotor blade 16. While epoxy adhesives are specifically identifiedherein, it should be appreciated that other binders, fasteners or otherremnants from manufacturing and/or assembly may similarly be disposedabout the wind turbine rotor blade 16 in non-uniform amounts therebyproducing variations in radar cross section by location.

Referring to FIGS. 2-4, the structural support member 50 may be disposedwithin the shell 40 between the leading edge 28 and the trailing edge 30and extend for at least a portion of the rotor blade span length 32. Thestructural support member 50 can comprise any supportive member that isdirectly or indirectly connected to and supporting the shell 40 and maycomprise one or more different materials.

For example, as illustrated in FIG. 3, in some embodiments thestructural support member 50 can comprise fiberglass. In suchembodiments, the structural support member 50 can comprise a spar 51 andone or more spar caps such as an upper spar cap 52 and a lower spar cap53. The spar 51, the upper spar cap 52 and the lower spar cap 53 mayextend for any length of the rotor blade span length 32 sufficient tosupport the overall wind turbine rotor blade 16. For example, in someembodiments the spar 51, the upper spar cap 52 and the lower spar cap 53may extend substantially the entire length of the rotor blade spanlength 32 from the root 20 to the tip 22. In some embodiments, the spar51, the upper spar cap 52 and the lower spar cap 53 may only extend fora portion of the rotor blade span length 32. In even some embodiments,the spar 51, the upper spar cap 52 and the lower spar cap 53 may extendfor different lengths independent of one another such as when the upperspar cap 52 and the lower spar cap 53 extend for a length beyond thespar 51 towards the tip 22. Moreover, while embodiments comprising thespar 51, the upper spar cap 52 and the lower spar cap 53 have beenpresented herein, it should be appreciated that other embodiments mayalso be provided for structural support members comprising fiberglasssuch as comprising only one of these elements and/or comprisingadditional elements not already described herein.

In other embodiments, such as that illustrated in FIG. 4, the structuralsupport member 50 may comprise a carbon fiber. In such embodiments, thestructural support member 50 may comprise a single spar 51 (i.e.,without the additional upper spar cap 52 and lower spar cap 53illustrated in FIG. 3) which comprises the carbon fiber material. Whilespecific materials have been presented herein, it should also beappreciated that additional and/or alternative materials may also beincorporated into the structural support member 50. Moreover, whileembodiments comprising the spar 51 have been presented herein, it shouldbe appreciated that other embodiments may also be provided forstructural support members comprising carbon fiber such as comprising anupper spar cap, a lower spar cap and/or additional elements not alreadydescribed herein.

Referring now to FIGS. 3 and 4, the wind turbine rotor blade 16 mayfurther comprise one or more cavities 60 internal the wind turbine rotorblade 16. The one or more cavities 60 can comprise voids in the interiorof the wind turbine rotor blade 16 that are not filled with structuralsupport members 50 or other components of the wind turbine rotor blade16. For example, in some embodiments there may be a leading edge cavity61 adjacent the leading edge 28 of the wind turbine rotor blade 16. Insome embodiments, there may additionally or alternatively be a trailingedge cavity 61 adjacent the trailing edge 30 of the wind turbine rotorblade 16. In other embodiments, additional and/or alternative cavities60 may also be present internal the wind turbine rotor blade 16 such asnear or around the structural support member 50. Moreover, in someembodiments, one or more of the cavities 60, such as the leading edgecavity 61 or the trailing edge cavity 62, may be divided into aplurality of sub cavities such that all or part of the cavity 60 mayfilled with radar absorbing materials as will become appreciated herein.

To assist in the reduction of the radar cross section of the windturbine rotor blade 16 at one or more locations, in some embodiments thewind turbine rotor blade 16 may comprise a lightweight broadband radarabsorbing filler material 70 disposed in at least one of the one or morecavities 60. The lightweight broadband radar absorbing filler material70 may comprise a relatively lightweight material that does notsubstantially affect the weight or balance of the wind turbine rotorblade 16 and is also capable of absorbing radar across a relativelybroadband spectrum such that the radar cross section of the wind turbinerotor blade 16 can be reduced for a plurality of frequencies. As usedherein, broadband spectrum refers to a frequency range spanning at leastabout 0.5 GHz, and in some embodiments at least about 1.0 GHz, such thatthe lightweight broadband radar absorbing filler material 70 is capableof reducing the radar cross section of at least a portion of the windturbine rotor blade 16 across a range in frequency.

The lightweight broadband radar absorbing filler material 70 cancomprise a foam matrix 71 and a plurality of carbon bodies 72 (orsimilar material having electromagnetic energy attenuationcharacteristics) dispersed throughout the foam matrix 71, such asprovided in a lossy foam. In some embodiments, the foam matrix 71 maycomprise open cell polyurethane foam that can support the plurality ofcarbon bodies 72. Such foam materials may allow for the dispersedincorporation of the electromagnetic energy attenuating material (e.g.,carbon bodies 72) while not weighing down the wind turbine rotor blade16 such that the wind turbine 10 would require redesign to maintain itsfunctionality.

The individual carbon bodies 72 may vary in their relative size, shapeand/or amount of electromagnetic energy attenuating material and may beeither uniformly or variably distributed throughout the foam matrix 71.For example, in some embodiments, the lightweight broadband radarabsorbing filler material 70 can comprise a variable carbon loadedmaterial wherein the amount of carbon in the lightweight broadband radarabsorbing filler material 70 is non-uniform. The non-uniformity of thecarbon loading may be a result of the number or concentration of carbonbodies 72 disposed in a certain region of the foam matrix 71, the sizeof the carbon bodies 72 disposed in a certain region of the foam matrix71, or combinations thereof. By varying the amount and/or location ofelectromagnetic energy attenuating material, the lightweight broadbandradar absorbing filler material 70 may reduce the radar cross section ofat least a portion of the wind turbine rotor blade 16 by attenuatingelectromagnetic energy across a broad spectrum. For example, in someembodiments, the lightweight broadband radar absorbing filler material70 can comprise one or more of the commercially available 320 series ofcarbon-based foam absorbers manufactured by Cuming Microwave.

The lightweight broadband radar absorbing filler material 70 may beincorporated into wind turbine rotor blades 16 comprising variousstructural support member 50 configurations and/or materials. Forexample, the lightweight broadband radar absorbing filler material 70may be disposed in one or more cavities 60 when the wind turbine rotorblade 16 comprises fiberglass (such as when it comprises a spar 51,upper spar cap 52 and lower spar cap 53 as illustrated in FIG. 3).Alternatively, the lightweight broadband radar absorbing filler material70 may be disposed in one or more cavities 60 when the wind turbinerotor blade 16 comprises carbon fiber (such as when it comprises asingle spar 51 comprising as illustrated in FIG. 4). While specificlocations and configurations of the lightweight broadband radarabsorbing filler material 70 have been identified herein, it should beappreciated that these are exemplary only; additional or alternativecombinations/configurations of wind turbine rotor blades 16 andlightweight broadband radar absorbing filler materials 70 should beappreciated.

Furthermore, still referring to FIGS. 3 and 4, the lightweight broadbandradar absorbing filler material 70 may be disposed in a variety oflocations in and around the wind turbine rotor blade 16 to reduce theradar cross section across a relatively broadband spectrum of radarfrequency as appreciated herein. For example, the lightweight broadbandradar absorbing filler material 70 can be disposed in at least one ofthe one or more cavities 60 of the wind turbine rotor blade 16.Depending in part on the size, shape and position of the specific cavity60, the lightweight broadband radar absorbing filler material 70 may beincorporated such that it fills just a portion of the cavity 60 (asillustrated in FIGS. 3 and 4), or fills the entirety of the cavity 60.In some embodiments, the lightweight broadband radar absorbing fillermaterial 70 may be disposed in a cavity 60 adjacent to wherever adhesiveis present. For example, the lightweight broadband radar absorbingfiller material 70 may be disposed in the leading edge cavity 61 and/orthe trailing edge cavity 62 such that the incorporation of a singlelightweight broadband radar absorbing filler material 70 can reduce theradar cross section of at least a portion of the wind turbine rotorblade 16 even though the amount of adhesive may vary by location or byblade.

The broadband aspect of the lightweight broadband radar absorbing fillermaterial 70 may thereby be utilized to reduce the radar cross sectionfor a variety of radar band frequency ranges (such as those used inNorth America and Europe) via the incorporation of a single element toprovide greater flexibility for where the wind turbine rotor blade 16 isdeployed. Moreover, the broadband aspect may also allow for thereduction in radar cross section of at least a portion of the windturbine rotor blade 16 when the local structure of the wind turbinerotor blade 16 requires the reduction of radar cross section across morethan a single frequency. For example, the variable amount of adhesiveutilized in the leading edge 28 and/or the trailing edge 30 of the shell40 may produce a radar cross section that varies based on adhesivethickness. A single lightweight broadband radar absorbing fillermaterial 70 may thereby assist in reducing the radar cross section ofthe entire adhesive structure despite its varying thickness.

Referring now to FIGS. 4-6, to assist in the reduction of the radarcross section of the wind turbine rotor blade 16 at one or morelocations, the wind turbine rotor blade 16 may alternatively oradditionally comprise a plurality of stacked resistive layers 80 (suchas for fiberglass blades as illustrated in FIG. 3) or may comprise asingle resistive layer 81 (such as for carbon fiber blades asillustrated in FIG. 4). As used herein, it should be appreciated that“resistive layer” includes both resistive layers and impedance layers.

Referring specifically to FIGS. 3 and 5, the plurality of stackedresistive layers 80 may be incorporated into wind turbine rotor blades16 having structural support members 50 comprising fiberglass. Theplurality of stacked resistive layers 80 may comprise two or moreindividual resistive layers 81, each separated by one or more ply layers82, which act in cooperation to absorb radar energy by converting it toheat. The plurality of stacked resistive layers 80 may be disposed atone or more locations about the structural support member 50 comprisingfiberglass and optionally the shell 40. Moreover, the plurality ofstacked resistive layers 80 may potentially be relatively tuned todifferent specific frequencies at different locations.

The plurality of stacked resistive layers 80 can comprise any pluralityof individual resistive layers 81 spaced to reduce the radar crosssection around one or more particular frequencies. In some embodiments,the individual resistive layers 81 may comprise, for example, a materialwith continuous carbon loading. In some embodiments, the individualresistive layers 81 may comprise circuit analog layers wherein thelayers comprise a radar absorbing circuit. Such circuit analog layersmay be capable of more precise tuning for absorbing radar of aparticular frequency. In some embodiments, the individual resistivelayers 81 of the plurality of stacked resistive layers 80 may comprise avariety of different types of resistive layers 81, such as where somecomprise circuit analog layers and other comprise material withcontinuous carbon loading. Moreover, the plurality of stacked resistivelayers 80 can comprise any number of individual resistive layers 81. Forexample, in some embodiments the plurality of stacked resistive layers80 may comprise 2 resistive layers 81. In some embodiments, theplurality of stacked resistive layers 80 may comprise up to 20 resistivelayers 81. The individual resistive layers 81 may be spaced apart atconstant or varying distances by one or more ply layers 82. Each plylayer 82 may comprise a thickness, such as about 10 mm, to separate theadjacent resistive layers 81. In some embodiments, the spacing may begreater or smaller and may depend on the size of the ply layer 82between the resistive layers 81.

Referring to FIG. 3, as discussed above, the plurality of stackedresistive layers 80 may be disposed in a variety of locations in andaround the fiberglass supported wind turbine rotor blade 16 to reduceits radar cross section across one or more frequencies. For example, inone embodiment, the plurality of stacked resistive layers may bedisposed at one or more locations about the structural support member 50of the wind turbine rotor blade 16. In such embodiments, the pluralityof stacked resistive layers 80 may be disposed such that they areintegrated with the structural support member 50 (e.g., alternatinglayers of fiberglass and resistive layers) and/or they may be disposedsuch that they are stacked on the exterior of the structural supportmember 50. Additionally, in some embodiments the plurality of stackedresistive layers 80 may be disposed at one or more locations about theshell 40 of the wind turbine rotor blade 16. In such embodiments, theplurality of stacked resistive layers 80 may be disposed such that theyare integrated with the shell 40 (e.g., alternating layers of fiberglassand resistive layers) and/or they may be disposed such that they arestacked on the exterior or interior of the shell 40. While specificlocations and configurations of the plurality of stacked resistivelayers 80 have been identified herein, it should be appreciated thatthese are exemplary only. Additional or alternative combinations and/orconfigurations of wind turbine rotor blades 16 and plurality of stackedresistive layers 80 should be appreciated.

Since the plurality of stacked resistive layers 80 may be tuned totarget specific frequencies (such as by varying the type of resistivelayer and/or the distances there between), a single wind turbine rotorblade 16 may have a reduced radar cross section for one or morefrequencies. For example, if it is known where the wind turbine rotorblade 16 will be deployed and what radar frequencies it will experience,than one or more plurality of stacked resistive layers may be disposedabout the wind turbine rotor blade 16 tuned for that particularfrequency. Conversely, if the wind turbine rotor blade 16 ismanufactured before it is known where it will be deployed, the specificfrequencies of incident radar may not be known. Therefore, in someembodiments, different pluralities of stacked resistive layers 80 may bedisposed about the wind turbine rotor blade 16 such that differentfrequencies can be targeted. For example, a first plurality of stackedresistive layers 80 may be disposed about the structural support member50 that is tuned for frequencies most prominent in North America. Asecond plurality of stacked resistive layers 80 may then be disposedabout the shell 40 that is tuned for frequencies most prominent inEurope. Thus, the same wind turbine rotor blade 16 may be deployed ineither geographical area and still possess a reduced radar cross sectionfor whichever frequency of radar it is subjected to. Such embodimentsmay streamline manufacturing and provide more versatile blades forbroader deployment.

Referring now to FIGS. 4 and 6, individual resistive layers 81 (withoutstacking) may be incorporated into one or more different locations ofwind turbine rotor blades 16 having structural support members 50comprising carbon fiber. The individual resistive layers 81 may comprisea resistive sheet disposed about the shell 40 and positioned to reducethe radar cross section with respect to the scattering that can occurfrom the large carbon fiber structural support member 50.

Specifically, in some embodiments, an individual resistive layer 81 maybe positioned between the structural support member 51 and the leadingedge 28. Such embodiments may allow for the absorption of radar energyscattered from the structural support member 51 proximate the leadingedge 28. Alternatively or additionally, in some embodiments, anindividual resistive layer 81 may be positioned between the structuralsupport member 51 and the trailing edge 30. Such embodiments may allowfor the absorption of radar energy scattered from the structural supportmember 51 proximate the trailing edge 30. Moreover, the individualresistive layers 81 may be disposed on the inside of the shell 40, onthe inside of the shell 40, or integral with the shell 40. For example,as illustrated in FIG. 6, the shell 40 may comprise a plurality of plylayers 82 such that the individual resistive layer 81 is disposedbetween two of said ply layers 82.

Furthermore, in some embodiments the individual resistive layers 81 maycomprise a tapered resistance such that the individual resistive layer81 has a lower resistance proximate the structural support member 50 anda higher resistance proximate the leading edge 28 or trailing edge 30.The tapered resistance can help absorb radar energy across a broaderrange to better reduce the overall radar cross section that can resultfrom the scattering off of the structural support member 51 comprisingcarbon fiber.

It should now be appreciated that lightweight broadband radar absorbingfiller material and/or one or more resistive layers may be incorporatedinto a wind turbine rotor blade to reduce its radar cross section. Theincorporation of such materials can provide radar cross sectionreduction over one or more frequencies while not imposing significantadditional physical constraints to the wind turbine rotor blade. Forexample, by incorporating one or more of the radar cross sectionreduction features disclosed and described herein, the radar crosssection of the wind turbine rotor blade may be reduced by at least 20dB, or potentially even 25 dB, to better allow for placement near oraround radar towers. Such wind turbine rotor blades may thereby beutilized in a variety of geographical regions (that use different radarbands) without requiring unique customization for its targeteddeployment.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

What is claimed is:
 1. A wind turbine rotor blade comprising: a shellcomprising a leading edge opposite a trailing edge; a structural supportmember that supports the shell and is disposed internal the wind turbinerotor blade between the leading edge and the trailing edge and extendsfor at least a portion of a rotor blade span length, wherein thestructural support member comprises fiberglass; one or more cavitiesinternal the wind turbine rotor blade; and, a lightweight broadbandradar absorbing filler material disposed in at least one of the one ormore cavities to provide the reduced radar cross section.
 2. The windturbine rotor blade of claim 1, wherein the lightweight broadband radarabsorbing filler material comprises a lossy foam.
 3. The wind turbinerotor blade of claim 1, wherein the lightweight broadband radarabsorbing filler material comprises a variable carbon loaded materialwherein the amount of carbon in the lightweight broadband radarabsorbing filler material is non-uniform.
 4. The wind turbine rotorblade of claim 1, wherein the leading edge comprises an adhesive and thelightweight broadband radar absorbing filler material is disposed in aleading edge cavity adjacent the leading edge.
 5. The wind turbine rotorblade of claim 4, wherein the adhesive comprises a non-uniformthickness.
 6. The wind turbine rotor blade of claim 1, wherein thetrailing edge also comprises an adhesive and lightweight broadband radarabsorbing filler material is also disposed in a trailing edge cavityadjacent the trailing edge.
 7. The wind turbine rotor blade of claim 1,wherein the lightweight broadband radar absorbing filler material atleast partially absorbs radar across a frequency range spanning at leastabout 0.5 GHz.
 8. The wind turbine rotor blade of claim 7, wherein thefrequency range spans at least about 1.0 GHz.
 9. The wind turbine rotorblade of claim 1, wherein the structural support member comprises a spardisposed between an upper spar cap and a lower spar cap.
 10. The windturbine rotor blade of claim 1 further comprising a plurality of stackedresistive layers disposed at one or more locations about the windturbine rotor blade to further provide the reduced radar cross section.11. The wind turbine rotor blade of claim 10, wherein the plurality ofstacked resistive layers wherein at least one of the plurality ofstacked resistive layers is disposed about the structural supportmember.
 12. The wind turbine rotor blade of claim 10, wherein theplurality of stacked resistive layers wherein at least one of theplurality of stacked resistive layers is disposed about the shell.
 13. Awind turbine rotor blade comprising: a shell comprising a leading edgeopposite a trailing edge; a structural support member that supports theshell and is disposed internal the wind turbine rotor blade between theleading edge and the trailing edge and extends for at least a portion ofa rotor blade span length, wherein the structural support membercomprises fiberglass; and, a plurality of stacked resistive layersdisposed at one or more locations about the structural support member toprovide the reduced radar cross section.
 14. The wind turbine rotorblade of claim 13, wherein the plurality of stacked resistive layerscomprise one or more ply layers disposed between each resistive layer.15. The wind turbine rotor blade of claim 14, wherein the structuralsupport member comprises the one or more ply layers such that theplurality of stacked resistive layers are integral with the structuralsupport member.
 16. The wind turbine rotor blade of claim 13, whereinthe plurality of stacked resistive layers consists of about 2 to about20 individual resistive layers.
 17. The wind turbine rotor blade ofclaim 13, wherein at least one of the plurality of stacked resistivelayers comprises a circuit analog layer.
 18. The wind turbine rotorblade of claim 13, wherein a first plurality of stacked resistive layersis disposed at a first location, and a second plurality of stackedresistive layers is disposed at a second location.
 19. The wind turbinerotor blade of claim 18, wherein the first plurality of stackedresistive layers at least partially absorbs radar in the 1 GHz to 2 GHzspectrum and the second plurality of stacked resistive layers at leastpartially absorbs radar in the 2 GHz to 4 GHz spectrum.
 20. The windturbine rotor blade of claim 18, wherein the first location comprisesthe structural support member and the second location comprises theshell.